This invention relates to providing pure sulfur tetrafluoride (SF4). More particularly, it relates to the removal of thiothionylfluoride (Sxe2x95x90SF2) from SF4 by contacting the impure SF4 with a high surface area material, such as activated carbon.
Sulfur tetrafluoride is an extremely useful fluorinating agent, which is particularly suited for the fluorination of oxygen-, sulfur- and nitrogen-containing groups bonded to non-metal or metal atoms. See, e.g., U.S. Pat. No. 3,950,498 to Appel et al. According to a publication of Air Products and Chemicals, Inc. (http://www.airproducts.com/fluorination/sulfur.asp; 2001), SF4 is particularly useful for deoxofluorinating aldehydes, ketones and carboxylic acids in the presence of hydrogen fluoride (HF) and other Lewis acid catalysts.
SF4 can be produced by a variety of methods. For example, U.S. Pat. No. 2,992,073 to Tullock describes a process for producing SF4 comprising reacting an alkali metal fluoride (wherein the alkali metal is Na, K, Rb or Cs) with a chlorine-supplying reactant (e.g., elemental Cl, etc.) and sulfur-supplying reactant (e.g., elemental S, etc.).
U.S. Pat. No. 3,950,498 to Appel et al. describes a process for the production of sulfur tetrafluoride, wherein sulfur tetrachloride or mixtures consisting of sulfur dichloride and chlorine are reacted with hydrogen fluoride in the presence of a carbon tetrahalide to provide SF4.
A problem with existing methods for manufacturing SF4 is the production of undesirable byproducts that contaminate the SF4. Such impurities can hinder the usefulness of SF4 as a fluorinating agent.
Thiothionylfluoride is a particularly undesirable SF4 contaminant. Thiothionylfluoride is typically observed in SF4 compositions at concentrations of 5% or more. Although thermally stable, Sxe2x95x90SF2 can undergo a disproportionation reaction to produce elemental sulfur when using SF4 as a raw material. The elemental sulfur byproduct can interfere with the SF4 chemistry and foul the processing systems.
Seel, xe2x80x9cLower Sulfur Fluoridesxe2x80x9d in 16 Adv. Inorg. Chem. Radiochem, 297, 303-304 (1974), describes a process for converting pure Sxe2x95x90SF2 to SF4 and elemental sulfur using catalysts, such as HF or BF3. Although Seel does not address the problem of removing Sxe2x95x90SF2 from SF4, the inventor is aware of a proprietary process for removing Sxe2x95x90SF2 from SF4, which employs the same chemistry as described by Seel to disproportionate Sxe2x95x90SF2 to form S8 and SF4 using HF (or BF3). However, this process is both inefficient and dangerous because it causes significant losses of SF4 and requires using large quantities of HF. The HF catalyst becomes deactivated during the disproportionation, because of the formation of the salt SF3+HF2xe2x88x92 (BF3 catalyst becomes deactivated by formation of the analogous salt with SF4). Thus, in order to have sufficient free HF present to initiate the disproportionation of Sxe2x95x90SF2, at least 30 mole % of HF must be mixed with the contaminated SF4. While this method produces SF4 free of impurities, about 20-25% of the SF4 is unrecoverable from the HF. The proprietary process is dangerous in that it involves handling large quantities of liquid anhydrous HF. HF is a gas under standard conditions, and reacts violently with many materials including glass. It can permeate large areas rapidly, and is extremely dangerous to humans. It is, therefore, desirable to avoid handling large quantities of HF.
The use of high surface area materials, such as activated carbon, zeolites, and silica, for separation processes is known. However, the inventor is not aware of any prior art disclosing the use of high surface area materials for removal of Sxe2x95x90SF2 from SF4.
Accordingly, it is desired to provide a safer and more efficient process for removal of Sxe2x95x90SF2 from SF4.
All references cited herein are incorporated herein by reference in their entireties.
The invention provides a process for removing thiothionylfluoride from a composition containing sulfur tetrafluoride and an initial concentration of thiothionylfluoride. The process comprises: contacting the composition with an activated carbon; reacting at least a portion of the thiothionylfluoride to form elemental sulfur; and recovering a purified composition containing the sulfur tetrafluoride and the thiothionylfluoride at a reduced concentration less than the initial concentration.
Also provided is a process for removing thiothionylfluoride from a composition containing sulfur tetrafluoride and an initial concentration of thiothionylfluoride, which process comprises: contacting the composition with a metal oxide-free material having a surface area of at least 400 m2/g; reacting at least a portion of the thiothionylfluoride to form elemental sulfur; and recovering a purified composition containing the sulfur tetrafluoride and thiothionylfluoride at a reduced concentration lower than the initial concentration.